1. Field of the Invention
The present invention relates to a non-contact and non-destructive measurement method of measuring film thicknesses of one or more transparent films of a sample object which is to be measured (hereinafter "sample object") in which the one or more transparent films are stacked on a substrate.
2. Description of the Background Art
To measure a film thickness of a transparent film of a sample object in which the transparent film is disposed on a substrate, a measurement method utilizing spectral reflectances is previously known. In this measurement method (hereinafter "first conventional method"), a sample object is irradiated with light of a certain observation wavelength range. Spectral reflectances for the sample object are measured to obtain an interference waveform. The number of peaks and valleys in the interference waveform and wavelengths corresponding to the peaks and valleys are then identified, and the film thickness d is calculated from these data. More specifically, the film thickness d is calculated as: ##EQU1## where: .lambda.1: the wavelength of a peak or a valley of the shorter wavelength side
.lambda.2: the wavelength of a peak or a valley of the longer wavelength side PA1 n1: the refraction index of the transparent film at the wavelength .lambda.1 PA1 n2: the refraction index of the transparent film at the wavelength .lambda.2 PA1 m: the total number of the peaks and the valleys in the observation wavelength range
However, if the sample object is formed by a plurality of transparent films which are stacked one atop the other on a substrate, it is impossible to measure the film thicknesses of the transparent films by the first conventional method. When light is irradiated upon the multilayered sample object and spectral reflectances are measured, interference within and between the transparent films are included in the measured spectral reflectances.
To deal with this, techniques for measuring film thicknesses of transparent films of a multilayered sample object have been developed in recent years. One example is a technique disclosed by U.S. Pat. No. 4,999,509 (hereinafter "second conventional method"). In the second conventional method, film thickness ranges for the respective transparent films are inputted in advance, and the film thicknesses of the transparent films of the multilayered sample object are calculated by global optimization and local optimization.
Thus, when a sample object includes only one transparent film disposed on a substrate, the film thickness of the transparent .film is measured by the first conventional method. On the other hand, when a sample object has a multilayered structure in which a plurality of transparent films are formed on a substrate, the film thicknesses of the respective transparent films are measured by the second conventional method.
Nevertheless, the first conventional method has a limited success in measuring a film thickness of a transparent film. Since a precondition for the first conventional method is that an interference waveform includes two or more peaks and valleys, if the interference waveform does not include peaks or valleys or the interference waveform includes only one peak or valley if not at all because the transparent film is relatively thin, it is impossible to measure the film thickness of the transparent film.
Neither the second conventional method is satisfactory since the second conventional method forces an operator an inconvenience of inputting film thickness ranges for the respective transparent films in advance for later global optimization. Although this inconvenience is avoided by inputting a sufficiently large film thickness range, such will instead increase the number of computation steps and unbearably lengthen a computation time. In addition, since a value calculated as a result of optimization largely varies depending on a starting point of optimization (i.e., the value of a film thickness of each transparent film) and other optimization parameters, a reproduction accuracy of measurement greatly drops depending on set conditions.
Although a combination of the first and second conventional methods makes it possible to deal with at least one or more transparent films of a sample object, this combination has a disadvantage that the film thickness measurement methods must be switched each other depending on the number of the transparent films at the expense of measurement process simplicity.